Blende sintering process



I LOUIS IBOSC Patented Oct. 20, 1931 UNITED STATES PATENT OFFICE HERON, F LIEGE, BELGIUM, ASSIGNOR TO SOGIETE ANONYME LA NOUVELLE MONTAGNE, OF ENGIS; BELGIUM, A COMPANY OF BELGIUM BLENDE SIN TERING PROCESS No Drawing. Application filedFebruary 21, 1930, Serial No. 430,398, and in Belgium March 9, 1929.

This invention relates to the roasting of blendes, and particularly to the roasting under forced draught, i. e. with air under pressure or drawn therethrough known as a sintering roast. During sintering as is known the temperature required is maintained. by

the combustion of a combustible element (sulphur in the form of sulphide, coke, etc.) contained in the ore or added thereto. In

* order to facilitate combustion and render it regular the ore is first of all reduced to size by crush ng, grlnding or preferably by agglomeration and d1v1s1on, or by extrusion in grams or nodules. Sintermg is, however, a

delicate operation, because it is very desirable to prevent local excess of temperature which would produce scorification or fusion of the ore. For this reason ores can only be treated which have been partially desulphurized, by a preliminary roasting, if desired with slight additions of raw blende or of coke. for-the purpose of maintaining combustion, the mean sulphur content of the material suitable for sintering hardly exceeding 810%. Moreover, these ores should be sintered in relatively thinlayers in order to prevent irregular propagation of the combuse tion. a

The present invention has for its object to g render possible the sintering of products 2.35 ofsuch thickness that the usual processes would be inapplicable in consequence of the intense scorification of the ore and of the faulty desulphurizing which would result.

' The invention is based on the observation '0 that the injurious rise of temperature produced when the mass to be roasted is of too great a height, or when the percentage of sulphur is too high, may be compensated effectively by developing during the roast- I ing operation an endothermic reaction, and

it has for its obje'ct a process utilizing for this purpose the decomposition of sulphate of zinc under the action of heat. According to this process the ore in divided form is 60 roasted under forced draught in a homogeneous mixture with a quantity of sulphate of Zinc determined according to the sulphur content of the ore, in such manner as to lower the temperature of roasting to a point which prevents scorification or fusion. It is true that in the course of the preliminary roasting by the usual processes a superficial sulphatation of the blende may be produced and that the small quantity of sulphate of zinc thus formed is then decomposed during the sintering, but this quantity which anyhow is not uniformly distributed throughout the mass, is quite insufiicientto produce a noticeable reduction in the roasting temperature with air under pressure or drawn therethrough. To obtain the result sought by the present invention it is necessary that the ore should contain a substantial proportion of sulphur in the form of sulphate distributed throughout the mass as will be clear from the following considerations.

The endothermic reaction of decomposition of the zinc sulphate counteracts the exothermic effect of the oxidation of a corresponding part of the sulphide of zinc. The theoretical proportion is about three molecules of sulphate of zinc to two molecules of sulphide. p

In the mixture to be sintered in accordance with the invention the total sulphur is distributed between the sulphide of zinc (sul-' phide-sulphur) and the sulphate of zinc (sulphate-sulphur). Of the sulphide-sulphur a part A should be used as in other processes to produce by its oxidation the heat necessary to maintain the reaction and to compensate for losses of heat. It plays the principal part in the combustion and may be called active sulphur. The remainder or part B of the sulphide-sulphur is the quantity which, if the ore were treated by The sulphide-sulphur constitute from the thermal aspect the inert sulphur. The preceding explanation may be indicated by the following diagram:

T t l sulphur sulphide-sulphur B If for example a blende be taken which in accordance with the usual processes should normally be preliminarily roasted till it contains at most 8% of total sulphur in order to avoid scorification on sintering, this same ore may be sintered in accordance with the invention without scorification when it contains much more sulphur (for example 20 to 25% total). This is obtained by so proportioning the amount of sulphate in the mixture that the quantity of sulphur which exceeds 8% be in the form of inert sulphur as explained above.

The above indicated figures are obviously theoretical. In the practical application of the invention these figures will be more or less modified to take account of various chemical and physical actions, capable of varying the thermal balance of the process particularly for the following reasons:

(a) The presence of sulphides other than zinc sulphide such as galena. pyrite, mis pickle, charcopyrite etc. or other constituents such as lime, baryta. silica etc. the mutual reactions of such materials. their thermochemical constants and their varying tendency towards scorification may slightly modify the result obtained by calculation based on pure blende.

b) A more or less pronounced state of hydration of the zinc sulphate and poss bl of other salts present. the dehydration of which would give rise to absorption of heat.

(0) The presence of a. more or less large quantity of zinc sulphate in the pre-roasted ore, whether such sulphate be intentionally left or produced by sulphatizing roast.

(d) The influence of the size of the grains and of the height of the mass in reaction, the present process permitting of a much greater sulphur content and of treating much greater thicknesses of material than is possible with the usual processes.

The sulphate of zinc employed in accordance with the invention can obviously come from any suitable source. It may for example be in the form of anhydrous or crystalline sulphate. It the latter case the endothermic reaction is even more accentuated by the dissoclation of the hydrated salt and the evaporation of the water of hydration. It may also advantageously be obtained from the treatment by means of water or sulphuric acid of waste products from reducing or roasting works. I may also employ the prodact of a sulphatizing roast, the product of reaction of a roasted blende, an oxlde of zinc, or zinc ores with sulphuric acid, the spent solutions coming from some electrolytic baths A-active sulphur sulphate-sulphur inertsulphur or other solutions of sulphate at suitable concentration.

When there is employed an anhydrous or partly dehydrated sulphate of zinc which is mixed with the moistened ore previous to granulation it is found that the sulphate is hydrated by absorbing the water in such manner that the grains and nodules can be obtained in a practically dry state, which permits of dispensing with, or at least reducing considerably the drying required. By the reaction of the sulphate and the oxides present in the mixture there may also be formed oxysulphat-es or basic sulphates of zinc more or less hydrated, the effect of which is to increase very greatly the hardening of the nodules.

When the sulphate of zinc is added in the state of solution, it may also be advantageously mixed with the ore previous to its granulation and act as a binding agent in the course of this operation. Such solution may be used in any suitable concentration and it can be obtained in a very economical manner by treating with water or with dilute sulphuric acid flue dust or other waste or by-products available in zinc works.

The mixture may also be prepared in a hot state with a quantity of water such that on cooling it allows a large part of the sulphate to crystallize in a very dispersed condition increasing the cohesion of the nodules, facilitating the distribution of the sulphate in the mass and diminishing the quantity of water necessary to ensure plasticity of the mass on granulation and consequently the water to be removed by drying.

To prevent premature action of the salts present there may be used a colloid, such as gum or dextrine. The gelatinous silica obtained for example in treating by sulphuric acid roasted ores or their residues may also be advantageously employed for this purthe draught necessitating excessive dilferences in pressure.

The roasting of the ore in layers of great thickness is moreover rendered possible with out abnormal rise of temperature as stated by the endothermic decomposition of the sulphate of zinc which is made at the expense of heat released by the combustion of a part of the sulphur present in the form of sulphide. This decomposition has other advantageous efiects. It frees sulphurous and sulphuric anhydrides, oxygens and water. The anhydrides on the one hand enrich the roasting gases so as to permit their employment for the manufacture of sulphuric acid under the same conditions as the gases coming from the ordinary roasting of raw blende. On the other hand the oxygen released and the sulphuric anhydride tend to oxidize the sulphide at once, in such manner that with the same result the speed of the air may be re duced, which contributes also to the enrichment of the roasting gases.

The process in accordance with the present invention is applicable not only to the sintering of pre-roasted blende, but it permits also of treating raw blende if the addition of zinc sulphate is sufficient and particularly if the sulphate employed is hydrated. It lends itself particularly well to the sintering of mixtures of preliminarily roasted blende and of raw blende and in this case permits of treating easily and economically mixtures much richer in raw blende than those required by the usual processes.

By way of example a pre-roasted ore having 6% of sulphur and giving by the usual sintering processes scorified agglomerations has been brought by the addition of raw blende and sulphate of zinc to form a mix ture containing 20% sulphur. This mixture with a thickness of 60 cms. has been subjected to sintering without scorification and has given a roast of about 0.45% of sulphur. The roasting gases contained about 9% sulphurous anhydride whereas the content of the gases obtained by the usual processes does not exceed 5%.

I claim:

1. In a blende sintering process, mixing the ore in a divided condition with zinc sulphate, proportioning the mixture in such manner that the sulphide-sulphur in excess of the amount necessary to ensure combustion is compensated by the presence of sulphate-sulphur, and sintering said mixture under forced draught.

2. In a blende sintering process, mixing zinc sulphide in a divided condition with zinc sulphate in the proportion of about three molecules of zinc sulphate to two molecules of zinc sulphide in excess of the percentage of zinc sulphide necessary to ensure combustion, and sintering said mixture under forced draught.

vided condition, the zinc sulphate being I present in a proportion sufiicient to prevent scoritication and fusion but insuflicient to check the maintenance of a suitable sinteringtemperature, granulating said mixture, and sintering the granulated mixture under forced draught.

5. In a blende sintering process, mixing Zinc sulphate with zinc sulphide and zinc oxide in a divided condition, the zinc sulphate being present in the proportion of about three molecules to two molecules of zinc sulphide in excess of the percentage of zinc sulphide necessary to ensure combustion, granulating said mixture, causing the formation of oxisulphate ofzinc within the mixture, and sintering the granulated mixture under forced draught.

6. In a blende sintering process, grinding the ore, mixing it with a solution of zinc sulphate, proportioning the mixture in such manner that it contains about threemolecules of zinc sulphate to two molecules of zinc sulphide in excess of the percentage of zinc sulphide necessary to ensure combustion, granulating said mixture and sintering the granulated mixture under forced draught.

7. In a blende sintering process, grinding the ore, preparing a zinc sulphate solution by the action of zinc containing waste products, mixing the ground ore With an amount of said solution sufficient to prevent scorification and fusion but insufficient to check the maintenance of a suitable sintering temperature, granulating the mixture,

and sintering the granulated mixture under forced draught.

8. In a process of treating blende, grinding the ore, mixing it in the presence of Water with zinc sulphate in the proportion of about three molecules of sulphate to two molecules of sulphide present in excess of the percentage of sulphide necessary to ensure combustion, granulating said mixture, and sintering the granulated mixture under forced draught.

9. I11 a process of sintering blende, grinding the ore, mixing the ground ore with zinc sulphate and with a colloid, the zinc sulphate being present in an amount suflicient to prevent scorification and fusion but insufficient to check the maintenance of a suitable sintering temperature, granulating said mixture and sintering the granulated mixture under forced draught.

10. In a process of sintering blende, prea: paring a mixture of raw blende and roasted 1 blende having a sulphide content above the 1 maximum content permissible for sintering without scorification and fusion under normal conditions, adding to said mixture an 3. amount of zinc sul hate sufficient to prevent scorification and usion but insufficient to check the maintenance of a suitable sintering temperature, granulating said mixture with the zinc sulphate in the presence of Water,

05 and sinterin the granulated mixture under forced draug t.

11. In a process of sintering blende, starting from a zinc sulphide ore in a divided condition having a sulphide content above 5 the maximum content permissible for sinter- 'mg without scorification and fusion under normal conditions, adding to said ore zinc sulphate in the proportion of about three molecules of sulphate to two molecules of B sulphide in excess of the amount of sulphide necessary for the maintenance of combustion, sintering the mixture and preventing undue rise of the sintering temperature by the endothermic decomposition of the sulphate pres- ID exit.

12. In a process of sintering blende, the step of subjecting to sintering under forced draught a mixture of zinc sulphide ore in a divided condition with zinc sulphate comprising sulphide in excess of the maximum percentage permissible for sintering without scorification and fusion under normal conditions and sulphate in suflicient amount to absorb by its endothermic decomposition only the excess heat above that necessary for maintaining a proper sintering temperature.

In testimony whereof I afiix my signature.

LOUIS BOSCHERON. 

